There's actually a few outcomes of the second stage that can occur (and some interesting tales to go along with them), but as geoffc has mentioned, second stage reuse is no longer planned for Falcon as Musk thinks the resources to develop it are better spent elsewhere. It's not an insurmountable technical challenge.
This is done for missions where the upper stage has enough remaining fuel reserves to ensure an intentional decay can occur safely. This has been done on every LEO mission since CRS-3 (including Orbcomm OG2), and usually results in the stage being deorbited Southsouthwest of Australia in the Indian Ocean (close to the area where MH370 was lost). We know this because occasionally SpaceX will post a NOTAM declaring the zone unsafe for a certain time. Here's the CRS-3 NOTAM, for example:
Left in GTO to decay
So far, this has been standard operating procedure for all 4 Falcon 9 upper stages that have delivered communications satellites to GTO. At this time, none remain in orbit, as the periapsis of each is so low (~200-300km) that they decay within 2-6 months. No deorbit profile has been attempted as it presents SpaceX with a liability if the stage decides to explode, scattering debris in GTO (additionally, if not enough fuel remains, they cannot safely deorbit either).
The standard apoapsis for GTO is about 35,000km, but for the first two GTO missions (SES-8 & Thaicom 6), they were injected into what is known as a "Supersynchronous Orbit" with an apoapsis beyond 35,000km. In the latter, it was in excess of 90,000km. It was launched on January 6 and decayed May 28.
You'd also expect the stage to disintegrate upon reentry, wouldn't you. Not always! The upper stage of AsiaSat 6 reentered over Brazil and parts of it were found scattered in an open field. It made quite the fireworks on reentry though:
Some of the largest items found intact were the Composite Overwrapped Pressure Vessels (COPV's), which stored helium.
Left in LEO to decay
This has been the case for Falcon 1 Flights 4 & 5 which remain in a nearly equatorial LEO to this day, as well as the first 5 Falcon 9 flights - of which COTS1 performed an unannounced upper stage restart boosting it into a 290x10,700km orbit & CASSIOPE, which attempted a "sideways" upperstage restart which failed, stranding it in a 900km polar orbit.
Solar (Heliocentric Orbit)
This hasn't happened yet, but the next mission, launching DSCOVR (currently scheduled for January 29, likely to be delayed), will see the upperstage place the 570kg payload in Earth-Sun L1, before entering a solar orbit, becoming the first piece of SpaceX hardware to leave Earth's orbit.